The invention relates to a device for generating images of an object of examination using nuclear magnetic resonance, in which coils are present for applying magnetic fields to the object of examination and for recording the displacement of the atomic nuclei of the object of examination from their equilibrium state by a high-frequency magnetic excitation pulse.
It is known that the nuclei of hydrogen atoms of the object under examination can be displaced from a preferred direction, by a high-frequency excitation pulse generated by a basic magnetic field, and that, due to their spin, the atomic nuclei return to the state of equilibrium in the preferred direction only after a specific time following the end of the excitation pulse. During this time, the atomic nuclei precess with a frequency which is dependent upon the strength of the basic magnetic field. If a field gradient is superimposed on this homogeneous basic magnetic field so that the distribution of the magnetic field varies in density, an orientation is possible via the respective measured frequency. It is also known that planographs of the object under examination can be prepared both in the above manner and by changing the direction of the field gradient. In this case, excitation in a section of the object under examination is achieved by influencing the basic magnetic field with an additional field gradient in such a manner that the atomic nuclei in this section are excited, since excitation occurs only at a frequency that is strictly assigned to the magnetic field in the desired section. Basically, only quantities such as those given below can be directly measured and graphically illustrated with a device of the type described in the preceding paragraphs: EQU S=c.multidot.f(.rho.,T.sub.1,T.sub.2)
In this equation, S is the measured signal and f is a function of the designated quantities which vary with the measurement procedure used. .rho. is the density and T.sub.1 and T.sub.2 are the relaxation times of the excited atomic nuclei. c is a constant that designates the sensitivity of the apparatus. By standardizing several sets of recorded data by using different recording parameters, it is basically possible to calculate absolute values for T.sub.1 and T.sub.2. The objective here is also to measure the directly accessible quantities of S in a reproducible manner and to measure with a sensitivity, and therefore a range, which is the same for different patients.
In practice this objective encounters a series of partially unavoidable effects which influence the sensitivity c of the measuring system and which therefore prevent the directly measured values from being reproducible and transferable. In particular, the different attenuation of the receive coil for patients of different diameters, for example, causes such an unavoidable change in the sensitivity of the measuring system. In practice, a calculation of absolute values for T.sub.1 and T.sub.2 will also be repeatedly incorrect due to the attenuation of the transmit pulses which vary from patient to patient, and result in incorrect high-frequency pulses. For standardization when using a device of the type previously described, a calibration device is usually provided with which calibration measurements can be made (GB 20 43 914). However, this calibration device is not suitable for use during the examination of a patient, because it can not be used for simultaneous measurements on the patient and for calibration measurements. It is also known that, in a device described in the beginning of this section, both a sample to be examined and a calibration sample must be provided (U.S. Pat. No. 3,501,688). In this device, however, there is no provision for simultaneously examining the patients and making calibration measurements.